The simplest way to determine the velocity of dissolution is to place the sample, a tablet for instance, in the solvent and to observe the changing of the solution's concentration. The samples are analysed e.g. with a spectrophotometer or by high pressure liquid chromatography. In order that comparable results might be achievable, the experimental conditions such as mixing and sampling must be closely standardized. It is not possible however by this method to achieve results with satisfactory repeatability. Errors are caused e.g. by the fact that the solvent volume decreases as samples are taken and the decrease has to be made up with new solvent; the location of the tablet in the container varies, with the result that the tablet is in different flow conditions at different times; it is difficult to draw the sample at exactly the same point; and small, solid particles of matter tend to be entrained in the sample.
In some methods of determination, a tablet of the substance to be examined is placed in a solvent container in a kind of rotating apparatus. For instance in the standard procedure of U.S. Pharmacopea XIX, the tablet is placed in a basket of a specified kind, this basket being rotated in the solvent under closely standardized conditions. The repeatability of results is poor with this method, too. The results present dispersion for the reason above all that the sampling point cannot be standardized. It is true, though, that the procedure can be improved by automation.
In perfusion methods, the tablet to be examined is placed in a chamber through which solvent is conducted. Following after the chamber, the solvent may be analysed e.g. in a flow-through spectrophotometer, whereby continuous information is gained on the dissolution. The procedure may be applied in the form of an open system (the differential method), in which case all the time new, pure solvent is fed into the chamber, or of a closed system (the integral method) wherein the same solvent is circulated. Even in the perfusion methods, result repeatability is poor.
Some velocity of dissolution determining methods have been comparatively assessed in: Bolhuis, G. K. et al., Pharmaceutisch Weekblad, 108, 49-53 (1973).
One of the weak points in procedures known in the art is also their inapplicability in the examination of powdery samples.